Formation of Divalent Ruthenacycles via Oxidative Cyclization of 1,3,5-Cyclooctatriene with Maleic Anhydride or Maleimides: An Intermediate for the Transition Metal-Mediated [6 + 2] Cycloaddition of 1,3,5-Trienes and Alkenes

Reactions of a zerovalent ruthenium complex, Ru(η4-cod)(η6-cot) (3; cod = 1,5-cyclooctadiene, cot =
1,3,5-cyclooctatriene), with maleic anhydride or maleimides under mild reaction conditions afforded a
series of novel divalent ruthenacycles 4 with an η5-cyclooctadienyl moiety via oxidative cyclization between
the carbon−carbon double bonds of cot and the electron-deficient alkene. The solid-state structure clearly
showed a newly constructed ruthenacycle skeleton, which was formed in a trans manner. Complex 4
was further reacted with H2 and HCl to give hydrogenated and protonated succinimides with a C8-ring,
respectively. When 4 was heated in toluene, a [6 + 2] cycloadduct of cot and a maleimide was obtained
via reductive elimination, which shows that a ruthenium-mediated stepwise [6 + 2] cycloaddition was
achieved. The addition of PPh3 to complex 4d promoted the reductive elimination, while bidentate
phosphines such as 1,2-bis(diphenylphosphino)ethane did not give the cycloadduct and stable ruthenacycles
11 were formed instead. Reactions of 4 with CO gave novel tricarbonyl ruthenacycles 12 along with
dissociation of the cod ligand, where neither reductive elimination nor CO insertion took place. The
results of a theoretical study were consistent with the idea that the energy barriers for reductive elimination
from ruthenacycles bearing cod or monophosphine ligands were lower than those from a ruthenacycle
bearing a diphosphine ligand. The formation of 12 was found to be more energetically favorable than
reductive elimination.